Ferroelectric liquid crystals and antiferroelectric liquid crystals are generally known as being liquid crystals that exhibit smectic phases. These liquid crystals are used in image-producing displays by utilizing the properties that both types of liquid crystals possess spontaneous polarization and that the direction of the spontaneous polarization changes under the influence of an external electric or magnetic field. It is reported by Clark et al. that liquid crystal electro-optical devices using ferroelectric liquid crystals have memory characteristics and are capable of fast response.
Ferroelectric liquid crystal is capable of exhibiting a plurality of optical states, and has the characteristic that it continues to retain a particular state even after removal of applied voltage. When an external force such as an electric field is applied, ferroelectric liquid crystal molecules lie in one of two stable positions on the lateral surface of a cone (liquid crystal cone). In a liquid crystal display device constructed by sandwiching such a ferroelectric liquid crystal between a pair of substrates, the ferroelectric liquid crystal is controlled so that the liquid crystal molecules lie in one of the two stable positions in accordance with the polarity of the voltage applied across the ferroelectric liquid crystal. One of the two stable states is called the first ferroelectric state, and the other is called the second ferroelectric state.
FIG. 1 shows one configuration example of a liquid crystal panel 20 that uses a ferroelectric liquid crystal 10. In FIG. 1, polarizers 15a (the direction of its polarization axis is designated by “a”) and 15b (the direction of its polarization axis is designated by “b”) are arranged in a crossed Nicol configuration. Here, the ferroelectric liquid crystal 10 is oriented so that the long axis direction of the liquid crystal molecules in the second ferroelectric state coincides with the polarization axis “a”. Accordingly, in the first ferroelectric state, the long axis direction of the liquid crystal molecules coincides with the direction of the other position on the liquid crystal cone.
When the polarizers 15a and 15b and the ferroelectric liquid crystal 10 are arranged as shown in FIG. 1, and the ferroelectric liquid crystal 10 is put in the second ferroelectric state by changing the polarity of the applied voltage (the long axis direction of the liquid crystal molecules in the ferroelectric liquid crystal 10 coincides with the polarization axis “a” of the polarizer 15a), light is not transmitted through the liquid crystal and the liquid crystal panel 20 thus produces a black display (non-transmission state). On the other hand, when the ferroelectric liquid crystal 10 is put in the first ferroelectric state by changing the polarity of the applied voltage (the long axis direction of the liquid crystal molecules in the ferroelectric liquid crystal 10 does not coincides either with the polarization axis “a” of the polarizer 15a or with the polarization axis “b” of the polarizer 15b), since the long axis direction of the liquid crystal molecules is tilted at a certain angle relative to the polarization axes, light, for example, from a backlight is transmitted trough the liquid crystal and the liquid crystal panel 20 thus produces a white display (transmission state). A light source other than the backlight may be used to produce the display.
Next, the switching of the ferroelectric liquid crystal 10, i.e., the transition from one ferroelectric state to the other ferroelectric state, will be described with reference to FIG. 2. As shown in FIG. 2, when the voltage applied to the ferroelectric liquid crystal 10 is increased, the voltage value at which light transmittance begins to increase is denoted by V1, and the voltage value at which the transmittance reaches saturation when the voltage is further increased is denoted by V2 (positive threshold). Then, when the voltage applied to the ferroelectric liquid crystal 10 is decreased, the voltage value at which the transmittance begins to drop is denoted by V3, and the voltage value at and beyond which the transmittance does not drop further even when the voltage is further decreased is denoted by V4 (negative threshold). Here, the state in which the transmittance is high corresponds to the first ferroelectric state, and the state in which the transmittance is low corresponds to the second ferroelectric state.
For example, when a voltage greater than V2 is applied to the ferroelectric liquid crystal 10, the ferroelectric liquid crystal transitions to the first ferroelectric state, and thereafter the ferroelectric liquid crystal retains the first ferroelectric state even when no voltage is applied, i.e., when 0 V is applied. Likewise, when a voltage greater in magnitude than V4 is applied to the ferroelectric liquid crystal, the ferroelectric liquid crystal transitions to the second ferroelectric state, and thereafter the ferroelectric liquid crystal retains the second ferroelectric state even when no voltage is applied, i.e., when 0 V is applied. In this way, once switched to a given ferroelectric state, the ferroelectric liquid crystal retains that state even after removal of the applied voltage. Such a ferroelectric liquid crystal is described, for example, in patent document 1.
In the liquid crystal panel using the ferroelectric liquid crystal, the molecules of the liquid crystal sandwiched between a pair of substrates each having an alignment film align themselves in a bookshelf- or chevron-like layer structure. The liquid crystal molecules can be driven parallel to the surfaces of the substrates by applying a pulsed electric field. When constructing a display device having a memory characteristic by using a ferroelectric liquid crystal, SiO alignment films are used.
FIG. 3 is a diagram showing the configuration of a liquid crystal cell 22 and the arrangement of polarizers when constructing a display using an antiferroelectric liquid crystal. The liquid crystal cell 22 is placed between the polarizers 21a and 21b arranged in a crossed Nicol configuration, the liquid crystal cell being set up so that the average long axis direction of the molecules in the absence of an applied voltage is substantially parallel to the polarization axis of either one of the polarizers and so that it can produce a black display when no voltage is applied and a white display when a voltage is applied.
FIG. 4 is a diagram showing the relationship between the applied voltage and the transmittance of the liquid crystal cell 22 constructed using the antiferroelectric liquid crystal. The voltage value at which the transmittance begins to change when the applied voltage is increased is denoted by V11, and the voltage value at which the transmittance reaches saturation is denoted by V12, while the voltage value at which the transmittance begins to drop when the applied voltage is decreased is denoted by V15. Further, when a voltage of opposite polarity is applied, the voltage value at which the transmittance begins to change when the absolute value of the applied voltage is increased is denoted by V13, and the voltage value at which the transmittance reaches saturation is denoted by V14, while the voltage value at which the transmittance begins to change when the absolute value of the applied voltage is decreased is denoted by V16.
As shown in FIG. 4, the first ferroelectric state is selected when the voltage is greater than the threshold of the antiferroelectric liquid crystal molecules. When the voltage of opposite polarity is applied, the second ferroelectric state is selected. In this way, in the antiferroelectric liquid crystal, when the voltage drops below a certain threshold from the ferroelectric state, an antiferroelectric state is selected. Such an antiferroelectric liquid crystal is described, for example, in patent document 2.
FIG. 5 is a diagram showing a region where the alignment tends to become unstable in a liquid crystal device constructed using a ferroelectric liquid crystal and an SiO alignment film. In FIG. 5, the ferroelectric liquid crystal injected through an injection hole 3 is sealed between two glass substrates 1 by a sealing member 2. Here, the region 6 is the region where the alignment tends to become unstable.
A smectic liquid crystal such as a ferroelectric liquid crystal is a mixture composed of a plurality of substances in prescribed proportions, and has a high viscosity. On the other hand, the SiO forming the alignment film is a porous material, in particular, when the film is deposited by evaporation, the surface is active. In this condition, when the smectic liquid crystal is injected through the injection hole, and a chromatographic phenomenon occurs between the liquid crystal and the alignment film, the substances forming the liquid crystal are adsorbed on the alignment film, and the composition of the liquid crystal gradually changes as the injection progresses. As a result, the composition of the liquid crystal becomes different between a region near the injection hole and a region far from it, and in the region far from the injection hole, the alignment state of the liquid crystal tends to become unstable, and display unevenness tends to occur.
Patent document 3 discloses a liquid crystal display device in which scanning electrodes are arranged in parallel to the direction of injection of the liquid crystal, with provisions made to apply a scanning signal from the side opposite to the injection hole in order to eliminate the display unevenness resulting from the unevenness of density of the liquid crystal material and to achieve uniform display quality. In patent document 3, it is described that highly polar components contained in the liquid crystal material are adsorbed on the alignment film during the injection of the liquid crystal, causing the density profile of the liquid crystal material to vary according to the distance from the injection hole. When the density profile of the liquid crystal material varies, a varying driving voltage profile is generated across the liquid crystal device such that the driving voltage is low in regions near the injection hole and high in regions far from it. Further, a voltage drop occurs due to the effect of the sheet resistance of the scanning electrodes, and the applied voltage varies according to the electrode position. In view of this, the liquid crystal display device disclosed in patent document 3 employs the configuration in which the scanning electrodes are arranged in parallel to the direction of injection of the liquid crystal and the scanning signal is applied from the side opposite to the injection hole. It is described that, with this configuration, the driving voltage profile resulting from the density profile of the liquid crystal material and the voltage drop resulting from the sheet resistance of the scanning electrodes compensate each other, and as a result, a proper scanning voltage can be applied and uniform display quality obtained.
Patent document 1: Japanese Unexamined Patent Publication No. 2006-23481 (FIGS. 1 and 2)
Patent document 2: Japanese Unexamined Patent Publication No. H10-239664 (FIGS. 2 and 3)
Patent document 3: Japanese Unexamined Patent Publication No. H4-355433 (Page 3, FIG. 2)